Integrated circuit designers have always sought the ideal semiconductor memory: a device that is randomly accessible, can be written or read very quickly, is non-volatile, but indefinitely alterable, and consumes little power. Emerging technologies are increasingly viewed as offering these advantages. Some nonvolatile or semi-volatile memory technologies include Magnetoresistive Random Access Memory (MRAM), Programmable Conductive Random Access Memory (PCRAM), Ferroelectric Random Access Memory (FERAM), polymer memory, and chalcogenide memory. Each of these memory types can be employed in stacked arrays of memory cells for increased memory density.
One type of MRAM memory element has a structure which includes ferromagnetic layers separated by a non-magnetic barrier layer that forms a tunnel junction. A typical MRAM device is described in U.S. Pat. No. 6,358,756 to Sandhu et al., entitled Self-Aligned Magnetoresistive Random Access Memory (MRAM) Structure Utilizing a Spacer Containment Scheme, filed Feb. 7, 2001. Information can be stored as a digital “1” or a “0” as directions of magnetization vectors in these ferromagnetic layers. Magnetic vectors in one ferromagnetic layer are magnetically fixed or pinned, while the magnetic vectors of the other ferromagnetic layer are not fixed so that the magnetization direction is free to switch between “parallel” and “antiparallel” states relative to the pinned layer. In response to parallel and antiparallel states, the magnetic memory element represents two different resistance states, which are read by the memory circuit as either a “1” or a “0.” It is the detection of these resistance states for the different magnetic orientations that allows the MRAM to read information.
A PCRAM memory element utilizes at least one chalcogenide-based glass layer between two electrodes. For an example of a typical PCRAM cell, refer to U.S. Pat. No. 6,348,365 to Moore and Gilton. A PCRAM cell operates by exhibiting a reduced resistance in response to an applied write voltage. This state can be reversed by reversing the polarity of the write voltage. Like the MRAM, the resistance states of a PCRAM cell can be sensed and read as data. Analog programming states are also possible with PCRAM. MRAM and PCRAM cells an be considered nonvolatile or semi-volatile memory cells since their programmed resistance state can be retained for a considerable period of time without requiring a refresh operation. They have much lower volatility than a conventional Dynamic Random Access Memory (DRAM) cell, which requires frequent refresh operations to maintain a stored logic state.
FERAM memory, another nonvolatile memory type, utilizes ferroelectric crystals integrated into the memory cells. These crystals react in response to an applied electric field by shifting the central atom in the direction of the field. The voltage required to shift the central atoms of the crystals of the cells can be sensed as programmed data.
Polymer memory utilizes a polymer-based layer having ions dispersed therein or, alternatively, the ions may be in an adjacent layer. The polymer memory element is based on polar conductive polymer molecules. The polymer layer and ions are between two electrodes such that upon application of a voltage or electric field the ions migrate toward the negative electrode, thereby changing the resistivity of the memory cell. This altered resistivity can be sensed as a memory state.
Chalcogenide memory switches resitivity states by undergoing a phase change in response to resistive heating. The two phases corresponding to the two resistivity states include a polycrystalline state and an amorphous state. The amorphous state is a higher resistive state, which can be read as stored data.
There are different array architectures that are used within memory technology to read memory cells. For instance, one architecture used is the so-called one transistor—one cell (“1T-1Cell”) architecture. This structure is based on a single access transistor for controlling read access to a single memory element. Another architecture is the cross-point architecture, where the read operation is performed without using an access transistor to control individual memory cells. This type of system uses row and column lines set to predetermined voltages levels to read a selected cell. Each system has its advantages and disadvantages. The cross-point system is somewhat slower in reading than the 1T-1Cell system, as well as being “noisy” during a read operation; however, the cross-point array has the advantage in that it can be easily stacked for higher density. Additionally, a 1T-1Cell array is faster, but necessarily less densely integrated than a cross-point array because additional space is needed to supply the 1-to-1 access transistor to memory cell ratio.
It would be desirable to have a memory read architecture that could utilize advantages from both the 1T-1Cell and cross-point architectures while minimizing the disadvantages of each.